Techniques for casting from additively fabricated molds and related systems and methods

ABSTRACT

According to some aspects, a method is provided of casting an object from a mold, the method comprising obtaining a mold comprising a hollow shell of rigid material, the material comprising a thermoset polymer having a plurality of pores formed therein, providing a metal and/or ceramic slurry into an interior of the mold, exposing at least part of the mold to a low pressure environment so that a net flow of gas is produced from the interior of the mold into the low pressure environment. According to some aspects, a method of forming a porous mold is provided. According to some aspects, a photocurable liquid composition is provided, comprising a liquid photopolymer resin, particles of a solid material, in an amount between 30% and 60% by volume of the composition, and a water-soluble liquid.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 15/981,279, filed May 16, 2018, titled “Techniques for Casting FromAdditively Fabricated Molds and Related Systems and Methods,” whichclaims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional PatentApplication No. 62/507,673, filed May 17, 2017, titled “Techniques forCasting From Additively Fabricated Molds and Related Systems andMethods,” and of U.S. Provisional Patent Application No. 62/507,661,filed May 17, 2017, titled “Photopolymer Composition,” each of which ishereby incorporated by reference in its entirety.

FIELD OF INVENTION

The present invention relates generally to systems and methods for theproduction of molds via additive fabrication, e.g., 3-dimensionalprinting, and to systems and methods for casting parts from such molds.

BACKGROUND

Gelcasting is a process in which a slurry of ceramic or metallic powdersand organic monomers is poured into a mold and then formed into a gel.To retrieve the gel part, the mold may be formed from a material that isdissolvable or otherwise easily removable. For instance, the mold may beformed from wax, which can then be melted or dissolved away. Once themold is removed, the gel can be dried and sintered to form a solidceramic or metal part.

Additive fabrication, e.g., 3-dimensional (3D) printing, providestechniques for fabricating objects, typically by causing portions of abuilding material to solidify at specific locations. Additivefabrication techniques may include stereolithography, selective or fuseddeposition modeling, direct composite manufacturing, laminated objectmanufacturing, selective phase area deposition, multi-phase jetsolidification, ballistic particle manufacturing, particle deposition,laser sintering or combinations thereof. Many additive fabricationtechniques build parts by forming successive layers, which are typicallycross-sections of the desired object. Typically each layer is formedsuch that it adheres to either a previously formed layer or a substrateupon which the object is built.

In one approach to additive fabrication, known as stereolithography,solid objects are created by successively forming thin layers of solidmaterial by curing portions of a liquid photopolymer. Exposure toactinic radiation cures a thin layer of the liquid photopolymer, whichcauses it to harden and adhere to previously cured layers or to thebottom surface of the substrate.

SUMMARY

According to some aspects, a method is provided of removing an objectfrom a mold, the method comprising obtaining an object formed within amold, the mold comprising a shell of rigid material that comprises athermoset polymer having a plurality of pores formed therein, and theobject comprising a solidified metal and/or ceramic slurry, applying asolvent to the mold, thereby softening the mold, and mechanicallyremoving the softened mold from the object.

According to some aspects, a photocurable liquid composition isprovided, comprising a liquid photopolymer resin, particles of a solidmaterial, in an amount between 30% and 60% by volume of the composition,and a water-soluble liquid.

According to some aspects, a method is provided of casting an objectfrom a mold, the method comprising obtaining a mold comprising a hollowshell of rigid material, the material comprising a thermoset polymerhaving a plurality of pores formed therein, providing a metal and/orceramic slurry into an interior of the mold, exposing at least part ofthe mold to a low pressure environment so that a net flow of gas isproduced from the interior of the mold into the low pressureenvironment.

According to some aspects, a method is provided of forming a porousmold, the method comprising forming, via additive fabrication, an objectcomprising a hollow shell of rigid material, wherein the materialcomprises a thermoset polymer and a plurality of solid particlesembedded within the polymer, removing at least some of the solidparticles from the object, thereby forming a plurality of pores in thehollow shell.

The foregoing embodiments may be implemented with any suitablecombination of aspects, features, and acts described above or in furtherdetail below. These and other aspects, embodiments, and features of thepresent teachings can be more fully understood from the followingdescription in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

Various aspects and embodiments will be described with reference to thefollowing figures. It should be appreciated that the figures are notnecessarily drawn to scale. In the drawings, each identical or nearlyidentical component that is illustrated in various figures isrepresented by a like numeral. For purposes of clarity, not everycomponent may be labeled in every drawing.

FIG. 1 is a flowchart of a method of casting a metal and/or ceramicobject from a fugitive mold formed via additive fabrication, accordingto some embodiments;

FIG. 2 is a flowchart of a method of casting a metal and/or ceramicobject from a porous mold, according to some embodiments;

FIG. 3 is a flowchart of a method of forming a porous mold via additivefabrication, according to some embodiments;

FIG. 4 is a flowchart of a method of removing a fugitive mold formed viaadditive fabrication, according to some embodiments;

FIG. 5 depicts an illustrative process of generating a three-dimensionalmodel suitable for fabrication as a fugitive mold, according to someembodiments;

FIG. 6 is a block diagram of a system suitable for casting a metaland/or ceramic object from a porous mold, according to some embodiments;

FIG. 7A depicts the structure of a fugitive mold formed via additivefabrication and having solid and liquid additives present therein,according to some embodiments; and

FIG. 7B depicts the structure of the fugitive mold of FIG. 7A afterremoval of the solid and liquid additives, according to someembodiments.

DETAILED DESCRIPTION

Various additive fabrication techniques may be applied to fabricateobjects having shapes suitable for use as molds. For instance, a hollowshell may be fabricated and used as a one-part mold, or two objects maybe fabricated that have been designed to be joined together and used asa two-part mold. In terms of the quality of the resulting cast, one-partmolds are generally preferable because multi-part molds can suffer frommisalignment of the molds and can produce mold lines in the resultingpart as a result of the seams between the molds. One-part molds canpresent a processing challenge, however, since the cast part must beretrieved from the mold after casting. For many one-part molds, at leastpartial destruction of the mold is inevitable due to the geometry of thepart making it impossible to remove the mold whilst leaving it intact.

Certain additive fabrication techniques utilize materials to fabricatemolds that may be readily dissolved in a suitable solvent. For example,additive fabrication that utilizes acrylonitrile butadiene styrene (ABS)to fabricate a mold will produce a mold that can readily be dissolved ina solvent such as acetone. Other additive fabrication techniques,however, may use materials which are less readily dissolved, oressentially insoluble in any suitable or practicable solvent, whichinhibits safe removal of a mold fabricated via such techniques withoutdamage to the part cast within it. In particular, additive fabricationtechniques that fabricate objects using thermoset materials, as opposedto thermoplastic materials, generally produce objects that cannot bedissolved in any conventional solvent. For instance, stereolithography,which applies radiation to a liquid photopolymer and thereby cures theliquid into a solid thermoset polymer, forms objects that are notdissolved in conventional solvents. In addition, objects so fabricatedfrom thermoset materials do not typically melt or otherwise thermallydegrade at temperatures allowing removal of the mold material withoutrisking damage to the part cast inside it. At the same time, manyprocesses capable of forming molds from such thermoset materials arecapable of improved surface finish, accuracy, and other desirablequalities.

The inventors have recognized and appreciated techniques for removal ofa thermoset polymer mold from a part cast within the mold by applying asolvent to the mold that will mechanically degrade, though notnecessarily dissolve, the mold. While, as discussed above, thermosetpolymers are not dissolvable in conventional solvents, the inventorshave recognized and appreciated that certain solvents will, over time,alter the mechanical properties of a thermoset polymer mold sufficientlyto allow mechanical removal of the mold from a part cast inside the moldwithout substantial risk of damage to the part. In particular, certainsolvents have been observed to cause a thermoset polymer mold to swelland soften, which allows the mold to then be peeled away from a partcast within the mold.

According to some embodiments, a part may be gelcast from a thermosetpolymer mold. In such cases, there may be a benefit to keeping the partat low temperatures during removal of the mold since, at that stage ingelcasting, the part has not yet been sintered and therefore may bestructurally stronger if held at a low temperature, which may includefreezing the part. As such, the application of a solvent to swell andsoften the thermoset polymer mold described above may utilize solvent ata low temperature so that the step of mechanically weakening the moldoccurs while the part is held at a low temperature. The mold can then beremoved whilst in the cold solvent, or after removal to a similarly coldenvironment.

Another challenge with molds, particularly one-part molds, is that voidscan be created when filling the mold with a casting material due to thecasting material not filling the entire interior of the mold. Inparticular, trapped air pockets can inhibit the flow of casting materialinto some regions of the interior, leading to defects in the cast part.One approach to addressing this issue is to produce small paths in themold through which air can escape but that are sufficiently small toinhibit the casting material from escaping, such as by poking holesusing a thin needle. In some cases, the mold can be filled with castingmaterial in a low pressure environment, which aids in evacuation of airfrom the interior of the mold and encourages the casting material tofill the mold. The process of forming paths in the mold through whichair can escape requires, however, manual application of a sharpimplement to the fabricated mold, which can cause unintended damage tothe mold and/or may be time consuming.

The inventors have recognized and appreciated techniques for fabricatinga porous thermoset polymer mold that exhibits an open cell network whichallows gas to pass through the mold whilst retaining casting materialswithin the mold. In some embodiments, a solid additive may be mixed witha liquid photopolymer to produce a composition from which a mold can beadditively fabricated using conventional additive fabricationtechniques, such as stereolithography. Subsequent to fabrication of themold, at least some of the solid additive can be removed from the mold,thereby producing a thermoset polymer mold having pores formed therein.The solid additive may be a powder such that fine particles areinterspersed throughout the fabricated mold. These particles may beremoved by dissolving, melting, or otherwise changing the physical stateof the particles such that they are separated from the mold.

In some embodiments, a non-photopolymer liquid additive may be mixedwith the liquid photopolymer and solid additive to produce a mixturethat has a desirable viscosity for additive fabrication. Without such aliquid additive, a mixture of liquid photopolymer and a solid additivemay have a viscosity that is sufficiently high that conventionaladditive fabrication processes that utilize the mixture as a sourcematerial either do not work successfully or suffer from undesirabledegradation of the quality of a fabricated mold. By including a suitableliquid additive, the viscosity of the mixture may be reduced so that thequality of a mold fabricated from the mixture is commensurate with thequality that would be expected when fabricating the same mold from onlythe liquid photopolymer.

Following below are more detailed descriptions of various conceptsrelated to, and embodiments of, techniques for the production of moldsvia additive fabrication and for casting from such molds. It should beappreciated that various aspects described herein may be implemented inany of numerous ways. Examples of specific implementations are providedherein for illustrative purposes only. In addition, the various aspectsdescribed in the embodiments below may be used alone or in anycombination, and are not limited to the combinations explicitlydescribed herein.

FIG. 1 is a flowchart of a method of casting a metal and/or ceramicobject from a fugitive mold formed via additive fabrication, accordingto some embodiments. At a high level, method 100 includes steps ofdesigning and fabricating a mold via additive fabrication (act 102),forming pores in the mold (act 104), casting a metal or ceramic part inthe mold whilst utilizing the pores to ensure the part substantiallyfills the mold (act 106), applying solvent to the mold to soften it (act108) and mechanically removing the softened mold from the part (act110). Subsequently, the part may be sintered or otherwise formed into afinal part. Each of these steps will be discussed in further detailbelow, and it will be appreciated that a suitable process may beenvisioned that performs some, but not all of the steps of illustrativemethod 100. For instance, the process of casting in act 106 may beperformed using a supplied mold (see FIG. 2), the process of fabricatinga porous mold in acts 102 and 104 may be performed without subsequentcasting steps (see FIG. 3), or the process of softening and removing amold from a cast part in acts 108 and 110 may be performed using asupplied cast part within a mold (see FIG. 4).

In the example of FIG. 1, in act 102 a mold is formed via additivefabrication. In some embodiments, the mold is formed of a thermosetpolymer via techniques such as, but not limited to, stereolithography orinkjet fabrication.

According to some embodiments, act 102 may include generation of acomputer model of the mold to be formed via additive fabrication. Insome embodiments, an object to be molded may be designed incomputer-aided design (CAD) software, scanned from an existing object,or otherwise created as a three-dimensional computer model. Based on thedesired geometry of the part, a comparatively thin shell may beautomatically generated enclosing the surface of the part (i.e., so thatthe part would be contained within the hollow interior of the shell).This shell, once fabricated, will serve as the mold structure insubsequent steps of method 100. In some embodiments, the dimensions ofthe part or shell may be adjusted to compensate for undesireddimensional changes that may occur during the casting process. Forexample, in gelcasting, depending on the specific slurry formulation andgeometry of the shell, the processing of slurry into a final part maycause substantial shrinkages, often between 15% and 20% from theoriginal molded form to the final part. As such, software mayautomatically compensate for the expected shrinkage by, for example,scaling the geometry of the part or shell globally by 15-20%. Either thepart may be modified to compensate prior to generation of the shell, orthe shell may be modified after its generation.

According to some embodiments, a shell may be automatically generated tohave a thickness that is selected based on the stiffness of the materialfrom which it will be fabricated. As an illustrative example, theinventors have fabricated a shell mold from the “Durable” photopolymerresin sold by Formlabs Inc. under the product number FLDUCL01. For thismaterial, the inventors have found a shell thickness between 0.5 mm and0.8 mm to be an effective thickness. Thinner shells may be too weak andflexible to fabricated whilst withstanding forces in subsequentprocesses of method 100. Thicker shells may be more difficult to removefrom the cast part in act 110, which may unnecessarily increase theprocess time. In some embodiments, software may be operated toautomatically generate a shell for a three-dimensional model of a partto be fabricated via additive fabrication, the shell having a thicknessbetween 0.2 mm and 1 mm, or a thickness between 0.4 mm and 0.8 mm, or athickness between 0.5 mm and 0.7 mm.

According to some embodiments, a computer-based model of a shell may beautomatically and/or manually modified in various ways, such as byadding reinforcing ribs, struts, and/or similar structures. Suchreinforcing structures may serve to limit global and/or localdeformation in the mold during later process steps of method 100. Insome cases, reinforcing structures may be advantageously tailored to theunique mold geometry using manual steps. Such manual customization,however, may involve undesirable user interaction and time, includingtraining in how a mold may deform under injection pressure. Accordingly,it may be desirable for software to provide various methods of guidedand/or fully automated reinforcement structure generation techniques.

In some embodiments, Boolean operations may be applied to thethree-dimensional model of the shell with a three-dimensional model of ageneric scaffolding geometry. The inventors have found that thefollowing characteristics may result in particularly effectivescaffolding structures. First, scaffolding components that are nothicker than 1 mm may aid in rapid demolding. Second, a scaffold that is“open-cell” may allow solvent to penetrate the mold more effectivelyduring demolding. Third, a distance between any two members within thescaffold of no more than 5 mm may ensure that the shell iswell-reinforced. The inventors have recognized that the desirablequalities of scaffolding for the shell bear similar characteristics todesirable qualities of structures for supporting additively manufacturedparts. As a result, techniques such as those described in U.S.application Ser. No. 14/543,138, titled “Systems and Methods ofSimulating Intermediate Forms for Additive Fabrication,” filed on Nov.17, 2014, which is incorporated herein by reference in its entirety, maybe useful for the development of scaffolding. In particular, forcesexpected to be caused by an injection molding process, includinginjection and/or demolding, may be considered by the simulationsdescribed in U.S. application Ser. No. 14/543,138, results of which maybe used to generate additional support structure and/or modify thegeometry of existing supporting structures.

According to some embodiments, a computer-based model of a shell may bemodified to add additional features that support processing of latersteps of method 100. As one example, a port or “sprue” shape may becoupled to the shell so that an opening extends outwards from the shell.Such a port opening may allow slurry to be injected into the moldthrough the port. In some cases, such a sprue may be most advantageouslypositioned at a location on the shell that allows for an evendistribution of slurry into the shell during injection.

In some embodiments, a sprue may comprise two parts. A first part of thesprue may form a plug that fits into and through a port on one side of avacuum chamber, forming a pressure tight seal between the sprue and theport walls. A second part of the sprue may form a port that may beconnected to a slurry injection source. In some cases, the second partmay be configured so that it can be attached pressure tight to theinjection source. In some embodiments, the sprue may be structured toinclude a standard luer lock fitting on the end of the sprue, allowingthe fitting to be attached to an injection source.

In some embodiments, multiple sprues may be added to a shell to formmultiple channels into the shell for injected material. The generationof multiple sprues in the shell may be advantageous should the shellincorporate internal restrictions or narrow portions that could restrictthe flow of the slurry through the interior of the shell.

According to some embodiments, vent holes may be automatically placed inthe three-dimensional model of the shell. With the case ofstereolithography or other additive manufacturing techniques that mayleave residue or material within the shell, vents may be added totemporarily allow for the removal of such unwanted material. In somecases, vent holes of approximately 1 mm in diameter may be generated inthe shell model so that, once the shell is fabricated, the vents can aidin a cleaning process that removes the residue (e.g., the IPA washingprocess used in stereolithography). In some embodiments, vent holes maybe advantageously placed to allow flow through all major parts of themold when washing solvent is injected (e.g., via the sprue). Prior toinjection of slurry, such vent holes may be sealed to prevent slurryfrom exiting the mold during casting. In some embodiments, the ventholes may be sealed prior to the molding process by application of afast curing glue, such as cyanoacrylate, and/or additionalphotopolymerizable resin to the holes.

Irrespective of whether the generated three-dimensional shell objectincludes reinforcing structures, a port/sprue and/or vent holes, theshell object may be manufactured in act 102 using any suitable additivemanufacturing technique(s). The inventors have found particular andunexpected advantages in forming such a part using stereolithographictechniques, such as those used by the Form 2 3D printer sold byFormlabs, to cure Formlab's “Durable” resin, available under productnumber FLDUCL01. As discussed further below, the selection of a materialfrom which a mold is fabricated may be made with respect to a givensolvent in order to achieve the desired degree of swell or othermechanical degradation of the mold prior to its removal. The materialmay also be selected so as to provide adequate structural support duringthe printing and casting processes with a minimum of thickness andmaterial.

In some embodiments, a composition from which the shell object isfabricated includes a liquid photopolymer combined with a solid additivethat is non-reactive with respect to the photopolymer. As will bedescribed further below, the solid additive may be in the form ofparticles that allow the shell object to be fabricated from thecomposition via conventional additive fabrication techniques, and whichcan then be removed from the object, at least in part, to produce poreswithin the shell. In some cases, both a solid additive and liquidadditive may be included in the liquid photopolymer composition. Anycombination of such additives described below may be used as a sourcematerial for fabricating the object in act 102.

As discussed above, a mold fabricated from a thermoset polymer may besoftened by application of a suitable solvent. While it may be desirableto maximize the degree to which the mold swells in the solvent, and thusyield easier shell mold removal, the same thermoset polymer propertiesthat result in substantial swelling, such as lower crosslink densitiesand lower glass transitions, may also be associated with lower stiffnessof the polymer. Lower stiffness, in turn, requires shell molds to befabricated with greater shell thicknesses and/or increased supportingribs or similar structures in order to compensate for the lowerstiffness. The inventors have appreciated that these contradictorydifficulties may be effectively addressed by the introduction of anadditional, embedded, stiffening material into the liquid photopolymerused to fabricate the shell mold. As such, according to someembodiments, rigid particles and/or fibers may be included within aliquid photopolymer composition from which a mold is fabricated in act102.

According to some embodiments, rigid particles, such as silica oralumina, may be incorporated into the liquid photopolymer compositionand thereby embedded into the final cured material in the shell mold.According to some embodiments, loadings of 10% to 30% by volume mayresult in increases in stiffness between 20% and 100%, depending on thestiffness of the particles and the original polymer. The particles mayhave a diameter between 1 μm to 20 μm range in order to promote gooddispersion, low viscosity, and allow printing of 25 μm to 100 μm layers.Such particles may not, however, substantially reduce the degree ofswell of the polymer-portion of the shell mold composite or increase theforces required to remove the shell mold after solvent exposure.

According to some embodiments, fibrous material, such as glass or carbonfibers, may be incorporated into the liquid photopolymer composition inrandom or controlled orientations in order to strengthen the shell mold.

According to some embodiments, particulate material with gaspermeability may be incorporated into the liquid photopolymercomposition to increase the permeability of the shell mold. As anexample, a composition may include a high volume fraction loading of agas permeable polymer powder, such as particles of PDMS, PMP, or anotherpolymer known to be gas permeable, thus increasing the gas permeabilityof the mold apart from perforations.

In some embodiments, once a fabricated shell part has been formed, postprocessing may be applied to the part in which the part is removed fromthe build platform and immersed into a bath of 90% or greater isopropylalcohol (IPA). The IPA bath may remove a significant portion of uncuredresin from the external surfaces of the shell. The part may be left inthe bath for approximately 10 minutes, for example.

In some cases, such a bath may not be sufficient to remove uncured resinfrom the interior surfaces of the shell, however. As such, in someembodiments a syringe or similar device may be attached to the shell(e.g., via one or more sprue connections) and negative pressure appliedthrough said syringe to draw IPA into and through the shell submerged inIPA via vent holes formed in the surface of the shell. This process maybe repeated multiple times to remove substantially all uncured resinfrom the interior of the shell. In some embodiments, the pressureapplied may be positive such that IPA is introduced through one or moresprue connections into the interior of the shell then exiting ordraining from the shell through the one or more vent holes mentionedabove. The inventors have found it advantageous to limited the totalexposure period to IPA, however (e.g., to no more than 20 minutes) inorder to avoid unwanted penetration and weakening of the shell at thisstage. Following the cleaning by IPA, the shell may be dried thoroughly,by the use of forced air and/or by heating (e.g., to a temperature of50-60° C. for approximately 15 minutes). Such elevated temperatures mayfurther serve to ensure completion of the curing process of the resinused and the evaporation of any IPA or other cleaning solvent from theinterior of the shell.

In act 104, pores may be formed within the mold fabricated in act 102.In some embodiments, small perforations may be added across the surfaceof the shell. These perforations may be produced in various ways,including by the manual use of a pin to mechanically pierce the shellwall. Such perforations are advantageously small enough so as tosubstantially contain the casting materials during a subsequent castingprocess, while remaining sufficiently large to allow gas to flow throughthem (e.g., during the evacuation of the mold when placed under vacuum).In some embodiments, such perforations may have a diameter of between 10μm and 50 μm, or between 15 μm and 40 μm, or approximately 25 μm.

According to some embodiments, the density of such perforations may beproduced based upon the geometry of the shell. As an example, certainregions, such as blind channels, high aspect ratio channels, and areaswhere there will be “knit lines,” may cause incomplete injection ofcasting materials. As such, these regions may be provided withcomparatively more perforations than other regions.

In some embodiments, the three-dimensional model of the shell may bemanually or automatically modified in act 102 in order to includefeatures allowing for the easier placement of perforations into theshell mold. As one example, small dimples or similar surface indicationsmay be formed directly into the surface of the mold guiding aperforating tool visually and/or mechanically to the desired locations.These perforations greatly increase the utility of stereolithographictechnology for the fabrication of shell molds, or indeed for anyfabrication process which typically results in a gas impermeable solid.In other additive manufacturing techniques, however, such as selectivelaser sintering or fused filament fabrication, the material formed bythe fabrication process may be inherently gas permeable and thus requirefewer, or no, perforations.

As discussed above, while the addition of the types of perforationsdiscussed above may have some utility, the inventors have recognized andappreciated techniques to form a gas-permeable shell mold usingstereolithography or similar processes from a liquid photopolymer resin.Indeed, if the shell mold is made from sufficiently permeable material,it may be possible to reduce the number of perforations or even avoidperforations altogether. This would be advantageous, as a highresolution and smooth surface finish of stereolithographic parts istypically a particularly desirable aspect of this additive manufacturingtechnique.

According to some embodiments, a permeable shell may be formed by theintroduction of one or more non-reactive additives (NRAs) to any one ofa number of additive manufacturing materials such that NRAs areincorporated within the final polymer matrix formed during thefabrication process. As discussed above in relation to step 102, theseadditives may be added to a composition comprising a liquid photopolymerand the mold fabricated in act 102 from said composition. As usedherein, a “NRA” refers to any compound that does not chemically bond toa polymer matrix produced during the additive fabrication process. Thus,a NRA is present in an initial composition from which a shell mold isfabricated, and subsequent to said fabrication, is not chemically bondedto the polymer matrix formed from said composition. Examples of relevantpolymer matrices include thermoplastics, such as those thermoplasticsknown to be useful in sintering, fused filament, or similar fabricationprocesses, or thermoset polymers, such as those thermosets known to beuseful in polymerization-based fabrication processes, such asstereolithography.

Once a shell mold is fabricated from a photopolymer and NRA composition(which may include other components as well), the NRA(s) may then beremoved via a secondary process. As one example, a NRA may be selectedfor preferential solubility, as compared to the polymer matrix, withrespect to a convenient solvent. Immersing the composite NRA and polymerobject into such a solvent may then result in the NRA dissolving intosolution, while leaving the polymer matrix structurally intact.Alternatively, or in addition, a NRA may be selected to react to athermal process, such that the NRA offgasses or leaches from the polymermatrix upon exposure to elevated temperatures. Provided that thestructural integrity of the polymer matrix is maintained, the removal ofNRAs from the polymer matrix using any of the above or relatedtechniques may result in a polymer matrix with numerous voids where NRAhad previously been incorporated into the matrix. This process maythereby produce a porous mold by producing a polymer network throughwhich gas may travel from the interior to the exterior of the mold, orvice versa.

In some embodiments, it may be particularly advantageous to fabricate amold containing an amount of a NRA sufficient that a large number ofdiscrete NRA domains are in contact or nearly in contact with otherdomains. The resulting voids, after removal of NRAs from the matrix,thus tend to form open cell networks throughout the matrix. In additionto providing pathways for gas permeability through the resulting porousmatrix, such open cell networks may also assist in the further removalof NRAs from the composite matrix by allowing solvent or other removalmaterials to penetrate more readily into the matrix, and for anyfugitive NRA materials to exit more readily.

The inventors have recognized and appreciated, however, that to producesuch an arrangement where a large number of NRA domains are in contactor nearly in contact with other domains, a substantial volume loading(e.g., greater than 40% by volume) of the solid NRA into the liquidphotopolymer precursor may be necessary. Such a degree of loading mayresult in significant increases in viscosity, which may be sufficientlyhigh to prohibit or inhibit the additive fabrication process using sucha composition. While some techniques may improve the suspension of thesolid NRA in the liquid photopolymer, these techniques typically involvepreventing the solid particles of NRA from coming into contact or“clumping,” and therefore may have the undesirable effect of preventingcontact between NRA particles in the final composite matrix, thusreducing the potential connections between cells in any resulting opencell network. Solid NRAs do, however, possess a number of useful anddesirable properties. For instance, the size of any eventual pores inthe resulting matrix can be substantially controlled by varying the sizeof the solid NRA particles introduced into the liquid photopolymerprecursor.

The inventors have recognized and appreciated that the limitationsdescribed above with respect to solid NRAs may be addressed by usingboth solid and liquid NRAs within the polymer matrix. Among otherbenefits, the combination of solid and liquid NRAs allow for thecomposite mixture to have a lower viscosity than solid NRAs alone wouldallow for, while having a higher rigidity than liquid NRAs alone wouldallow. Moreover, the combination of the solid and liquid NRAs may allowfor more free contact between the solid and liquid NRA domains. This isparticularly true wherein the solid and liquid NRAs have similarchemical affinity, such as hydrophilic properties. This is believed tocreate, in effect, bridges or connecting domains of liquid NRA betweensolid NRA particles, even where both the liquid NRA and solid NRAcomponents are individually well dispersed. The result is the creationof well-connected and thus porous open cell networks within a fabricatedmold.

As with solid NRAs, when used alone, a large volume fraction of liquidNRA, often 40-50% by volume, would be required to form the desired opencell network. This large volume fraction of liquid NRA may have anunwanted plasticizing effect, making the cured material softer andweaker. In addition, as with the solid NRA, techniques commonly used forimproving the stability and microphases of the liquid NRA within theliquid photopolymer precursor may compromise the ability to form aconnected open pore network, by preventing regions of liquid NRA fromcoming into contact in the polymer matrix.

In either case of using only liquid NRAs or only solid NRAs, it may bedifficult to provide a material with acceptable properties in itsprecursor state while achieving highly porous open cell networks in thefinal product. In part this is because particles within the polymermatrix are unlikely to touch one another at anything other than thehighest concentration. In the case of using solid NRAs, this is becausea practical photopolymer resin containing solid NRAs has the particles“well dispersed,” such that solid NRA particles have been mixed into themonomer matrix in such a way that they are not touching each other butare in fact completely surrounded by monomer. This process has the aimof slowing the settling of these particles, as well as reducing theviscosity of the resin. However, this means when the resin is cured, theparticles will not be touching, which is not conducive to the formationof an open cell network. This may be solved by having a very highloading of solid NRAs, in order to make the distance between particlesas small as possible. This has the aforementioned disadvantages ofmaking the formulation very viscous. When using liquid NRAs alone thereis a similar issue. The liquid NRA is phase-separated from the polymermatrix, which is again achieved by ensuring the liquid NRA is welldispersed such that the microphases are not contacting each other. Inview of the above, there are distinct advantages to using both solid andliquid NRAs in a photopolymer composition.

According to some embodiments, a composition from which an object may befabricated may comprise a liquid photopolymer, a solid NRA and a liquidNRA. The amount of the liquid photopolymer may comprise at least 20%, atleast 30%, at least 35%, at least 40%, at least 42%, or at least 43% ofthe composition by volume, and may comprise up to 60%, up to 55%, up to50%, up to 48%, or up to 47% of the composition by volume. According tosome embodiments, combinations of the above-referenced ranges are alsopossible. For instance, according to some embodiments, the amount of theliquid photopolymer may comprise an amount between 30% and 60% by volumeof the composition, may comprise an amount between 40% and 55% by volumeof the composition, or may comprise an amount between 43% and 47% byvolume of the composition. In some embodiments, the liquid photopolymermay comprise an amount approximately equal to 45% by volume of thecomposition. The amount of the solid NRA may comprise at least 20%, atleast 30%, at least 35%, at least 40%, at least 42%, or at least 43% ofthe composition by volume; and may comprise up to 60%, up to 55%, up to50%, up to 48%, or up to 47% of the composition by volume. According tosome embodiments, combinations of the above-referenced ranges are alsopossible. For instance, according to some embodiments, the amount of thesolid NRA may comprise an amount between 30% and 60% by volume of thecomposition, may comprise an amount between 40% and 55% by volume of thecomposition, or may comprise an amount between 43% and 47% by volume ofthe composition. In some embodiments, the solid NRA may comprise anamount approximately equal to 45% by volume of the composition. Theamount of the liquid NRA may comprise at least 2%, at least 3%, at least5%, at least 7%, at least 8%, or at least 9% of the composition byvolume; and may comprise up to 20%, up to 18%, up to 15%, up to 12%, orup to 11% of the composition by volume. According to some embodiments,combinations of the above-referenced ranges are also possible. Forinstance, according to some embodiments, the amount of the liquid NRAmay comprise an amount between 3% and 20% by volume of the composition,may comprise an amount between 8% and 15% by volume of the composition,or may comprise an amount between 9% and 12% by volume of thecomposition. In some embodiments, the liquid NRA may comprise an amountapproximately equal to 10% by volume of the composition.

According to some embodiments, the solid NRA may comprise particleshaving a mean diameter between 50 μm and 100 μm, or between 65 μm and 85μm, or approximately 75 μm.

As one illustrative example of a suitable composition, a formulation ofphotopolymer resin was prepared by combining, by weight, 3 parts UVcurable monomer, 1 part polyethylene glycol (PEG) (molecular weight400), and 6 parts sodium bicarbonate (NaHCO₃). The UV curable monomerserves as the polymer matrix, and is curable using radiation cureadditive manufacturing technologies. The NaHCO₃ may serve as a solid NRAthat may be removed by the application of an acidic solvent, and PEGserves as a liquid NRA.

According to some embodiments, the liquid NRA may be selected fromcompounds with an incompatible affinity to the polymer matrix material,while retaining an affinity to the solid NRA particles and any chosensolvent for the removal of the NRAs. For example, various siloxane oilsand/or propylene glycol may be a suitable substitute for PEG.

In some embodiments, the solid NRA may be a material that significantlyincreases in volume and/or decompose to produce a significant amount ofgas when a solvent is applied to the shell mold. In particular, theinventors have found that the use of particles of NaHCO₃ to beparticularly effective as a solid NRA. Such particles may be removed byuse of an acidic liquid, such as diluted acetic acid. As will beappreciated, particles of NaHCO₃, being basic, will react with theacetic acid and produce carbon dioxide gas and a soluble salt. Theincrease in volume due to the gas evolution may help to force materialfrom the polymer matrix, thus accelerating the NRA removal process.Moreover, the forces generated may be sufficient to compromise thinnerregions of polymer matrix separating particles of the solid NRA, thusincreasing the connectedness of the open cell network within theresulting porous material. Other components may also be chosen as solidNRA, either on their own or in combination with one or more solid NRAs.In particular, certain compounds are known to be gas evolving solids,known as “blowing agents.” Such compounds, provided that they do nothave an affinity for forming bonds with the polymer matrix, areappropriate for use as solid NRAs, including azodicarbonamide. Some suchcompounds require chemical reactions with a second chemical, such as anacidic salt NRA reacting with a basic solvent, while others may beactivated in other ways, such as thermal treatment. Indeed, those havingskill in the art will appreciate that NaHCO₃ has a further mechanism ofaction whereby gas may be produced in response to heating, without thepresence of an acidic solvent. This dual-action nature may beparticularly effective in causing extensively connected open cellnetworks, even in regions of the material where an acidic solvent haslimited or no initial penetration.

In embodiments where a liquid NRA is incorporated into the polymermatrix, it is desirable that the liquid NRA be immiscible in the polymermatrix, such that the liquid NRA phase separates into microphases in thepolymer matrix. These microphases are micro droplets within the polymermatrix composed of the liquid NRA. When using liquid NRA's, the eventualpore size may be controlled by altering the size of the microphasesincorporated into the polymer matrix. This may be done in a number ofways, including using different mixing methods, changing the ratio ofimmiscible components, using multiple immiscible components to create avariety of pore sizes, or adding surfactants and/or dispersants tomodify the miscibility of the components.

In the example of FIG. 1, in act 106 a metal or ceramic part may be castfrom the mold fabricated in act 102 and in which pores were formed inact 104. In some embodiments, the part may be cast from the mold usinggelcasting techniques, which are described in detail below. At a highlevel, however, a slurry (referred to below as a “feedstock”) may besupplied into the mold, which is then allowed to cool and set. Theslurry may contain a gelling agent that, when mixed with the othercomponents of the slurry, causes the slurry to set into a gel whencooled. Alternatively, some other component of the slurry may be presentwithin it that changes state when cooled sufficiently to set the part.In some embodiments, vacuum assisted casting techniques may be used toforce a highly loaded metal or ceramic feedstock slurry into the voidwithin the shell.

A wide variety of potential feedstocks may be utilized as castingmaterials in act 106, such as metal or ceramic slurries. In someembodiments, a feedstock may comprise a first and second component thatsolidify the feedstock within the mold. A first solidification componentfor the feedstock may have a temperature dependent characteristic suchthat a reduction of temperature causes the feedstock to solidify. Inparticular, the inventors have found feedstocks that are substantiallywater-based to be particularly advantageous for this purpose, such thatthe water within the feedstock may be frozen into ice crystals attemperatures below the freezing point of the water carrier within thefeedstock. A second solidification component may be included within theslurry that acts to solidify the slurry in a manner that is notdependent on temperature. As one example, gelling agents such as agar orgelatin-based agents may be included in the slurry in order to react andcontribute to the solidification of the slurry.

According to some embodiments, it may be advantageous for a feedstock tocontain a high content of metal or ceramic particles, the amount ofwhich is sometimes referred to as the “loading” of the slurry. Further,it is important that the slurry be of sufficiently low viscosity toallow it to flow into the mold without leaving unwanted voids. Finally,it is desirable that the slurry be capable of solidifying within themold such that the cast part formed of solidified slurry may be removedand processed in subsequent steps. According to some embodiments, avolume loading of the solid (metal or ceramic) component may be between40% to 55%. Particle sizes of the solid component may be between 5 μm to50 μm, and a distribution of particles sizes in the feedstock mayincrease the packing density of the particles during the sinteringprocess. Larger particles may have an advantage of producing a lowerviscosity slurry, however may resulting in a less dense sintered part.The powder composition may be, for example, a metal (such as aluminum ortungsten)/metal alloy (such as stainless steel or tungsten carbide) or aceramic compound (such as silica or alumina).

According to some embodiments, a solvent component of the feedstock maybe a liquid compound that enables the feedstock to easily flow duringthe mold filling process. Water may be particularly advantageous as ithas excellent properties when frozen and has a very low viscosity,allowing high loadings of solid particles to be used while maintainingreasonable slurry viscosity. Water also readily forms gels in a numberof methods, and is less volatile than some other solvents, such asalcohols. According to some embodiments, the solvent may representbetween 30% and 50% of the volume in the feedstock.

According to some embodiments, the feedstock may comprise a bindingagent. The binding agent may be any of a number of compounds that allowsthe feedstock to transition from easily flowing slurry to a solid or gellike state during the demolding process. One illustrative binding agentis a thermoplastic gelling agent, such as a polysaccharide (e.g.,gelatin or agar) or a similar synthetic polymer. These gelling agentswork by forming a continuous molecule throughout the solvent, forming agel. This gel can be reduced to a liquid upon heating, and reformed intoa gel by cooling. Another illustrative binding agent is a thermosetcross linking monomer. These agents are composed of a monomer(s) thatare capable of reacting to form a crosslinked polymer within thesolvent. The reaction is initiated either chemically or photochemically,and is distinct from the thermoforming gel binding agent in that it isirreversible. Another illustrative binding agent is an inorganic bindingagent, such as a silica based binding agent, that is soluble in thesolvent while the solvent is liquid, and that becomes insoluble when thesolvent is frozen, causing the binding agent to precipitate and bind toitself, forming a gel. Another illustrative binding agent is the solventitself. For example, a feedstock using water as the solvent may befrozen to form a solid feedstock. Freeze drying techniques may then beused to remove the water without melting the feedstock, resulting in apart ready for sintering. According to some embodiments, a binding agentmay comprise between 5% and 15% percent of the feedstock by volume.

As an illustrative embodiment, a feedstock may comprise approximately50% by volume stainless steel 17-4PH powder with a particle size ofapproximately 10 μm, water as a solvent and agar as a binding agent.This feedstock is a rigid gel at room temperature, a low viscosityslurry at 60° C., and a hard solid at freezing temperatures. Sinteredparts had a volume density of 95-98%, with 18% shrinkage. The slurry wasobtained from United Materials Technologies, LLC, Newark N.J. 07103.

In act 106, according to some embodiments, a slurry may be cast into amold by the following process. First, a quantity of feedstock may beheated to an elevated temperature prior to use. Such heating maydecrease the viscosity of a wide range of potential feedstocks and allowfor the more rapid and effective injection of the feedstock into themold. In some embodiments, heating the slurry feedstock to a temperatureof 60° C. in an oven may be sufficient. The mold formed by the shell maythen be placed into a vacuum-assisted injection chamber, such that themold is placed within the chamber with one or more sprue structuresextending through ports in the chamber to the outside of the chamber.These ports may then be connected, through a pressure tight valve, to acontainer for the injection of feedstock, such as a syringe orpressurized container. With the valve initially sealed, the vacuumchamber may then be depressurized, such as by exposure to a hard vacuumsource. Generally the higher the vacuum the better, with good resultsobtained with >28 mmHg vacuum pressures. Lower vacuum pressures may alsobe acceptable, though with increased risk of bubbles or other voidsremaining in the mold after the injection of the feedstock. Gasescontained within the shell mold may be vented through the perforationsdescribed above into the vacuum chamber and then exhausted as thechamber pressure is lowered and reaches equilibrium. The valve to thefeedstock container may then be opened, such that the vacuum pressurewithin the chamber, and within the shell mold, draws feedstock into theshell mold with significant force. Additional pressure may be applied tothe feedstock container, compressing any bubbles or voids formed byoutgassing. Once the injection process has been completed, and the shellmold is filled, the vacuum may be released.

According to some embodiments, it may be advantageous to further coatthe shell mold prepared as above with an additional reinforcing layer onthe interior or exterior of the shell mold. Ideally, such an additionallayer would comprise a gas permeable material. Alternatively, theperforations described above may be applied after the coating of theshell mold, so as to cause the perforations to also render theadditional layer gas permeable. In some embodiments, mixture of 20 partssand, 1 part water, and 1 part polyvinyl acetate (PVA) glue forms aneffective material to form an additional supporting coating onto a shellmold. Such a mixture may be used as an investment material, surroundingthe shell mold, or may be adhered onto the surfaces of the shell moldwith an adhesives. The mixture advantageously remains gas permeableafter setting, allowing for the removal of gasses from the shell mold,as described below, including through vacuum venting perforations in theshell mold. When an adhesion, rather than investing, application ofadditional material is used, it may be advantageous to apply theadditional layer material as a thin liquid, so as to allow the shellmold to be dipped and coated by the material in an effective manner.When such material is desired to be applied to only the exterior of themold, the sprue and venting holds may be sealed or blocked during theapplication. Alternatively, if only the interior of the mold is desiredto be coated, the coating material may be introduced via the sprue orventing holes into the interior and drained via the same channels. Sucha technique may be particularly useful for the application of anydesired release agents, such as silicone oil or other materials into theinterior of the shell mold.

After injection, the slurry within the shell mold may be solidified andneed to be removed from the shell mold. In embodiments of the presentinvention, the gel-like solidification mechanism within the slurry mayhave reacted after the injection process, thus forming a solidified orpartially solidified feedstock within the shell mold, sometimes referredto as a “blue part.” This blue part, however, may have very poormechanical properties and thus be at high risk of breaking or distortingduring the removal of the shell mold and any subsequent steps. This isparticularly true for certain geometrical features, such as high aspectratio extensions or finer details. Embodiments of the present inventionovercome this difficulty, and others, by reducing the temperature of thesolidified feedstock to below the freezing point of thetemperature-sensitive solidification system. In some embodiments, thisis advantageously the process of freezing the water-based slurry into asolidified composite of ice, slurry solids, and any other additives.This frozen blue part may possesses significantly improved mechanicalproperties as well as resistance to solvents.

In act 108, the mold may be softened to enable mechanical removal fromthe frozen blue part. As discussed above, molds formed from thermosetpolymers cannot be dissolved by conventional solvents, but the inventorshave recognized and appreciated that certain solvents may nonethelesssoften the polymers over time to such an extent that the polymers can beremoved from the blue part.

According to some embodiments, in act 108 the blue part, possiblyalready frozen, may be immersed in a bath of solvent held at atemperature below the freezing point of the temperature-sensitivesolidification system. This solvent advantageously penetrates thestructure of the shell mold materials causing it to substantially weakenor otherwise degrade in mechanical stability to a point where it may beeasily removed by hand, or with the assistance of suitable tools, fromthe frozen surface of the blue part within the shell mold. This removalprocess is made substantially easier and more effective by maintainingthe blue part at the lowered temperature, thus increasing its strengthand resistance to any deforming forces that may be applied during theremoval of the shell mold material. In embodiments described above usingFormlabs-brand products, the inventors have found immersing the shellmold and encased blue part into a bath of acetone held below 0° C. to beparticularly effective.

Without intending to be limited to a specific theory, the inventorstheorize that acetone resin penetrates into the cured “Durable”photopolymer material forming the shell mold, causing the polymermaterial to undergo an expansion referred to herein as solvent swellingor simply “swell.” Such swelling, in turn, substantially decreases themechanical properties of the photopolymer material forming the shellmold such that previously stiff and unyielding cured material may betransformed into a soft, easily tearable material suitable for removalusing comparatively little force. In general, materials formed usingphotopolymer materials are within a class of polymers known as thermosetpolymers. Such thermoset polymers may undergo greater solvent swellingto the extent the material has a lower crosslink density and/or a lowerglass transition temperature. In turn, increased solvent swellingtypically results in decreases in mechanical properties and thus easierremoval of shell mold material.

Irrespective of how the mold is softened in act 108, in act 110 the moldmay be mechanically removed, which may include manual peeling and/ortearing away of the shell material once degraded by the solvent.

According to some embodiments, a shell mold may be immersed in a chilledsolvent bath then taken out of the bath for removal of the shellmaterial after it has been sufficiently degraded by the solvent. In someembodiments, the shell mold may be removed from the chilled solvent bathand placed into another environment with reduced temperature, such as awater bath or cold chamber. The latter approach may be preferable insome cases, so as to maintain the frozen state, and subsequentstability, of the blue part, during the removal of the shell moldmaterial. The de-molded blue part may be said to be in a frozen gelstate, retaining the majority of the feedstock solvent (e.g., water).Once the blue part is removed from the mold, it may be allowed to warmto room temperature, or above, such that the feedstock solvent mayevaporate. During this stage, the non-temperature dependentsolidification component provides the blue part with sufficientstructural stability to avoid deformation or other damage. However, asthe warming and evaporation process may be conducted without physicalinsult to the blue part, the required mechanical properties aresignificantly less than those for other steps. Once the feedstocksolvent has evaporated, the remaining components of the feedstockmaterial have typically formed a rigid and easier to handle form,sometimes known as a “green part.”

Such green parts may then be further processed using techniques ofdebinding and sintering. In some embodiments, the green part may beplaced into a furnace and a debinding and sintering heating scheduleapplied to the feedstock material, such as those schedules for metalinjection molded (MIM) 17-4PH parts. This process generally consists ofholding the green part at an elevated temperature while any remainingsolidification agents are volatilized and outgassed from the green part.At this stage the part undergoes the first major step of shrinkage. Theresulting part is often known as a “brown part”. Next the furnace may beramped to a sintering temperature at which point the sintering and finalshrinkage/densification occurs, resulting in the final, nearly fullydense part. Embodiments of the present invention are capable of yieldingparts with densities>95%.

According to some embodiments, additional supporting structures may beincorporated into the furnace during the debinding and heating processin order to help counter any potential deformation due to gravity,shrinkage stresses, or other distortions. Some geometries are known tobe more at risk of such distortions—for example, high aspect ratiofeatures and cantilevered geometries. These supporting structures,sometimes referred to as “firing furniture”, are structures thatphysically support the part during debinding and firing to avoid thesedistortions. Firing furniture may be made out of the same material asthe part it is supporting. In doing so, the intended result is that thefurniture shrinks the same amount, and at the same time, as the part itis supporting. Such firing furniture has, heretofore, typically beengenerated manually for each specific green part geometry by a manualprocess. The process resulting in the formation of a shell mold, above,includes a geometrical description of the surface of the part. Suchgeometry may be readily used in order to automatically generate firingfurniture corresponding to the green part. As an example, the volume ofthe green part may be subtracted or removed, such as by geometricalBoolean operations, from a base shape of potential firing furniture,such as a rectangular prism of sufficient dimensions to encompass thegreen part. Alternatively, the base shape may contain voids, such as anarea filled with a scaffolding structure akin to the supportingscaffolding discussed above. Once the geometry is calculated, the“furniture” part may be converted into a shell mold and fabricated asabove, with the intended final part placed onto the resulting furnitureduring the casting. Alternatively, the geometry of the firing furnituremay be fabricated more directly utilizing a UV curable ceramic slurry.Several UV curable ceramic slurries have been demonstrated to print oncommercially available stereolithography printing platforms. Suchslurries are designed to go through a two-stage firing process, with alower temperature debinding step followed by a higher temperaturesintering step, similar to the approach above. Moreover, such slurriesmay be optimized to obtain percentages of shrinkage as close as possibleto those of the primary feedstock, so as to preserve the match ingeometrical distortion during the firing process.

FIG. 2 is a flowchart of a method of casting a metal and/or ceramicobject from a porous mold, according to some embodiments. Method 200 isan illustrative example of act 106 discussed above in relation to FIG.1, according to some embodiments. In the example of FIG. 2, a porousmold is obtained in act 202 and a metallic/ceramic slurry provided intothe mold in act 204. In act 206, the porous nature of the mold isexploited by positioning the mold in a low pressure environment so thatthe slurry is able to fill the mold and the presence of voids in theresulting cast is mitigated. It will be appreciated that acts 204 and206 may occur simultaneously (e.g., the slurry may be provided to a moldalready in a low pressure environment) in some embodiments.

FIG. 3 is a flowchart of a method of forming a porous mold via additivefabrication, according to some embodiments. Method 300 is anillustrative example of acts 102 and 104 discussed above in relation toFIG. 1, according to some embodiments. In the example of FIG. 3, acomposition of a liquid photopolymer and solid NRA is obtained in act302. In some embodiments, the composition may also include a liquid NRAas discussed above. In act 304, a mold object is additively fabricatedfrom the composition, such as via stereolithography. In act 306, thesolid NRA is removed from the mold object via any of the techniquesdiscussed above (e.g., reacting a NaHCO₃ solid NRA with an acid).

FIG. 4 is a flowchart of a method of removing a fugitive mold formed viaadditive fabrication, according to some embodiments. Method 400 is anillustrative example of acts 108 and 110 discussed above in relation toFIG. 1, according to some embodiments. In the example of FIG. 4, anobject cast within a mold produced via additive fabrication is obtainedin act 402. In act 404, the mold is softened by application of asolvent. For example, a mold formed from a thermoset polymer may besoftened by acetone, as discussed above. In act 406, the mechanicallycompromised mold may be mechanically removed from the object.

FIG. 5 depicts an illustrative process of generating a three-dimensionalmodel suitable for fabrication as a fugitive mold, according to someembodiments. The steps of FIG. 5 may be performed by a suitable softwareapplication, which may perform the illustrated steps automatically whenprovided with a three-dimensional model of an object to be cast asinput.

In the example of FIG. 5, an object to be cast is represented in step510 by both an exterior view (upper) and a cross-sectional view (lower)of the same object. As discussed above, in step 520, suitable softwaremay automatically generate a hollow shell object based on the object ofstep 510. In particular, the hollow interior of the hollow shell mold issuch that the object to be cast would fit inside it. Since the shell hasa finite thickness, this naturally results in an increase in the totalvolume of the object relative to the object of step 510. As discussedabove, in some embodiments, the shell may be increased in scale by thesoftware to counteract the expected effects of shrinkage during castingand sintering of the object from the mold.

In the example of FIG. 5, reinforcing structures in the form of ribs areautomatically added by the software to the hollow shell mold in step530. As discussed above, these ribs may increase the structuralintegrity of the hollow mold without altering the interior of the shell.In step 540, a sprue component is attached to the ribbed shell mold ofstep 530. As discussed above, the sprue may both provide a suitable portfor injection of feedstock into the mold interior and may provide apressure seal when disposing the mold in a low pressure environment byproviding a plug between the parts of the object that are inside andoutside of the low pressure environment. FIG. 6 provides an example ofhow such a sprue might be utilized in this manner. In the example ofsystem 600, a shell mold 610 including a sprue portion is positionedthrough a port into a vacuum chamber 620. The body of the shell mold islocated in the low pressure environment of the chamber, but part of themold (the sprue portion 615) extends outside of the chamber to a valve630. The valve may be actuated to supply feedstock from syringe 640 intothe shell mold. If the mold is porous, any air within the shell mold mayescape into the vacuum chamber, allowing the feedstock to substantiallyfill the mold.

According to some embodiments, system 600 may be operated in analternate manner to perform a hydraulically reinforced casting process.In such a process, chamber 620 may be filled with an oil or other liquidand sealed off, such that when pressure or vacuum is applied to theshell mold 610 via the valve 630 the liquid-filled chamber resistsdeformation of the mold. An illustrative and non-limiting injectionprocess is as follows.

After the shell mold 610 is mounted into chamber 620, the chamber may befilled with a liquid, which may include a vegetable oil, silicone oiland/or a vacuum oil (which may or not be a silicone-based vacuum oil).In some cases, the liquid-filled chamber may be degassed. A vacuum maythen be applied to the shell mold through the valve 630 to evacuate themold. As discussed above, the liquid in the chamber may resist anydeformation of the mold that might otherwise result from saidevacuation. A pressurized source of feedstock may then be switched withthe vacuum to quickly fill the mold. For instance, a syringe offeedstock may be attached to an outlet of the valve 630 and compressedair applied to the syringe. The valve 630 may be switched from thevacuum source to the feedstock outlet, which fills the mold withfeedstock. After the feedstock in the mold sets (which in some cases mayinclude cooling of the feedstock), the liquid can be drained fromchamber 620 and the mold removed.

FIG. 7A depicts the structure of a mold formed via additive fabricationand having solid and liquid additives present therein, according to someembodiments. As discussed above, solid and liquid NRAs (711 and 712,respectively) may form a network within the cured photopolymer 715 of afabricated object, an example of which is shown in FIG. 7A. Once theNRAs are removed (techniques for which are discussed above) an open cellstructure results, as shown in FIG. 7B.

While the proceeding inventive disclosure is provided with embodimentsdirected to the casting of metal, it should be appreciated that theinventions herein may be applied across a wide range of domains andshould not be so limited. As one example, an additively fabricated shellmaterial that is permeable has been described. Such permeable materialshave multiple applications and the utility of permeable molds has beendemonstrated. As one example, the machining of thermoforming molds outof permeable material may be performed to improve the level detailachievable in thermoforming, as well as eliminating the need to addpinholes to the mold, which result in small surface defects in the finalproduct. Injection and blow molds are sometimes machined from permeablematerials to reap similar benefits. Additively fabricated molds arealready used in the thermoforming industry, and to a lesser extent inthe injection and blow molding industry. An additively fabricatedpermeable material has clear potential utility in these industries.Moreover, permeable materials manufactured according the techniquesabove may be impregnated with alternative materials, potential withoutor without vacuum assistance, to achieve composite materials withimproved mechanical, thermal, electrical, or chemical properties. Inaddition, permeable materials may have utility in industries thatrequire custom high surface materials, such as filtration, catalysis,bio-scaffolding, energy storage, and more.

Moreover, it should be appreciated that although certain photopolymercompositions have been described herein in the context of gelcasting,such compositions may have utility in other contexts. As such, thephotopolymer compositions described herein should not be viewed as beinglimited to any particular uses or applications.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated that various alterations,modifications, and improvements will readily occur to those skilled inthe art.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andscope of the invention. Further, though advantages of the presentinvention are indicated, it should be appreciated that not everyembodiment of the technology described herein will include everydescribed advantage. Some embodiments may not implement any featuresdescribed as advantageous herein and in some instances one or more ofthe described features may be implemented to achieve furtherembodiments. Accordingly, the foregoing description and drawings are byway of example only.

Various aspects of the present invention may be used alone, incombination, or in a variety of arrangements not specifically discussedin the embodiments described in the foregoing and is therefore notlimited in its application to the details and arrangement of componentsset forth in the foregoing description or illustrated in the drawings.For example, aspects described in one embodiment may be combined in anymanner with aspects described in other embodiments.

Also, the invention may be embodied as a method, of which an example hasbeen provided. The acts performed as part of the method may be orderedin any suitable way. Accordingly, embodiments may be constructed inwhich acts are performed in an order different than illustrated, whichmay include performing some acts simultaneously, even though shown assequential acts in illustrative embodiments.

Further, some actions are described as taken by a “user.” It should beappreciated that a “user” need not be a single individual, and that insome embodiments, actions attributable to a “user” may be performed by ateam of individuals and/or an individual in combination withcomputer-assisted tools or other mechanisms.

Use of ordinal terms such as “first,” “second,” “third,” etc., in theclaims to modify a claim element does not by itself connote anypriority, precedence, or order of one claim element over another or thetemporal order in which acts of a method are performed, but are usedmerely as labels to distinguish one claim element having a certain namefrom another element having a same name (but for use of the ordinalterm) to distinguish the claim elements.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having,” “containing,” “involving,” andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

What is claimed is:
 1. A method of casting an object from a mold, themethod comprising: obtaining a mold comprising a hollow shell of rigidmaterial, the material comprising a thermoset polymer, the hollow shellhaving a plurality of pores formed therein; providing a metal and/orceramic slurry into an interior of the mold; exposing at least part ofthe mold to a low pressure environment so that a net flow of gas isproduced from the interior of the mold into the low pressureenvironment.
 2. The method of claim 1, wherein a mean size of theplurality of pores is between 20 μm and 100 μm.
 3. The method of claim2, wherein the mean size of the plurality of pores is between 40 μm and60 μm.
 4. The method of claim 1, wherein said exposing at least part ofthe mold to a low pressure environment comprises disposing the at leastpart of the mold in a chamber and lowering an air pressure within thechamber.
 5. The method of claim 4, wherein the air pressure is loweredto below 0.1 atm.
 6. The method of claim 4, wherein the mold includes asprue portion, and wherein disposing the at least part of the mold in achamber comprises enclosing portions of the mold other than the sprueportion inside the chamber such that the sprue portion extends frominside to outside of the chamber and forms a pressure seal between thesprue portion and the chamber.
 7. The method of claim 6, whereinproviding the metal and/or ceramic slurry into the interior of the moldcomprises providing the slurry through the sprue portion into theportions of the mold other than the sprue portion.
 8. The method ofclaim 1, wherein the thermoset polymer is an acrylic.
 9. The method ofclaim 1, wherein the mold comprises a plurality of ribs formed on itsexterior surface.
 10. The method of claim 1, wherein the hollow shellhas a thickness between 0.5 mm and 0.8 mm.
 11. The method of claim 1,further comprising removing from the interior of the mold an objectformed from the metal and/or ceramic slurry.